1. Field of the Invention
The present invention pertains to apparatus for detecting leaks in an annular ring, and more particularly to apparatus for detecting leaks in a packoff seal positioned between an oil or gas wellhead and an inner tubular member which is suspended from the wellhead.
2. Description of the Prior Art
Crude oil and natural gas wells drilled offshore often have their wellheads positioned on the ocean floor. The wellhead is normally installed by mounting it on the upper end of a string of casing which is lowered into the bore hole and cemented in place. The wellhead is generally in the shape of a cylinder having a bowl-shaped bore which is adapted to sealably support an inner concentric string of casing and a casing hanger that is affixed to the upper end of the inner casing string prior to lowering the string and seating the hanger within the bowl of the wellhead. The casing hanger has an enlarged diameter, and is adapted to be received and supported within the bowl of the wellhead. The casing hanger is provided with a packoff element around its outer periphery to provide a pressure tight seal between the outside of the casing and the inside of the wellhead. The casing hanger also has a bowl shaped inner bore similar to that of the wellhead, and is adapted to support the next string of pipe or casing. In this fashion, a nest of casing hangers may be supported one atop the other within a wellhead, with each of the casing hangers serving to suspend a successively smaller concentric string of casing.
In many present day well completion systems annular packoff seal elements are positioned between the wellhead and each casing hanger to provide a pressure-tight seal therebetween. It is important that gas, water and other fluids which may be forced into some of the outer casings be prevented from moving into the inner casings, and that fluids from the inner casing be prevented from moving into the outer casings. Each of the packoff seals must be designed to withstand the maximum well pressure since pressure abnormalities are controlled by closing the blowout preventers situated above the wellhead, and this will cause high pressure to be exerted across the various seals. It is, therefore, important to test each of the packoff seals after it has been installed to be sure that it will withstand any pressure that may be encountered during operation of the well.
Prior art packoff seal testers employ a test tool which is lowered by the drill string into position on the upper portion of the inner casing. A seal assembly closes the upper bore of the string of inner casing, and the blowout preventers are then closed around the drill string. Fluid is then introduced under pressure into the wellhead beneath the blowout preventers. The pressure is increased until the differential pressure across the packoff seal reaches the desired test level. Leakage in the packoff seal is detected by monitoring the amount of fluid that must be supplied to maintain the differential pressure across it. If the wellhead is filled with fluid and flow of fluid into the wellhead is terminated, it is assumed that there is no leakage and that the seal is good.
The above technique has the disadvantage that, for it to detect a leak in the packoff seal, fluid must be free to flow across the defective seal at a detectable rate. If the space which is normally beneath the seal being tested is partially filled with cement or mud, a faulty seal may not permit sufficient volume of fluid to leak through for this to be detected by the fluid flow equipment. An additional difficulty with this procedure is that the outer casing normally has a lower bursting rating than the inner casings so that, if the pressure is increased rapidly and the seal is faulty, the possibility exists that the testing procedure will cause the outer casing to burst.
Another prior art packoff seal testing method is disclosed in the U.S. Pat. No. 3,872,713 to Ilfrey et al. In this method a detector assembly having a plurality of feeler arms is inserted into the bore of the inner casing just below the packoff seal. The bore of the inner casing is plugged to prevent pressure in the space above the plug from causing fluid flow down the inner casing. The blowout preventers are then closed to provide an isolated space above the packoff seal, and fluid under pressure is applied to the space immediately above the packoff seal. If any of the fluid leaks through the packoff seal, this fluid produces an inward pressure on the outside of the inner casing causing the dimensions of the inner casing to change slightly, and this slight change in dimensions is then noted by the feeler arms of the detector assembly. One of the disadvantages of this method of detecting a leak in the packoff seal is the fact that there is very little change in the dimensions of the inner casing due to an increase in pressure on the outside of a casing, and these changes may not be readily detectable especially for small leaks in the packoff seal.